1. Field of the Invention
The present invention relates to a torque sensor, and more particularly to a torque sensor suitable for use in a vehicle power steering system and the like.
2. Description of Related Art
A power steering system which provides steering assisting power for a vehicle incorporates a torque sensor that detects a torque exerted on the steering shaft when the steering wheel is turned; based on the torque detected by the torque sensor, a motor is actuated to assist the steering force.
FIG. 1 is a block diagram showing the configuration of a prior art torque sensor. An oscillator 110 produces an oscillating voltage which is passed through a buffer circuit 113 and applied to a torque detecting coil 114 and a temperature compensating coil 115 through respective voltage dividing resistors R.sub.0 and R.sub.0. The torque detecting coil 114, which is mounted in encircling relationship to a steering shaft (not shown), is so constructed that its impedance changes according to the torque exerted on the steering shaft and to the ambient temperature. An AC voltage generated in the torque detecting coil 114 is supplied to clamp circuits 118 and 119. The temperature compensating coil 115, which is also mounted in encircling relationship to the steering shaft, is so constructed that its impedance changes according to the ambient temperature. An AC voltage generated in the temperature compensating coil 115 is supplied to clamp circuits 116 and 117.
Output voltages of the clamp circuits 116 and 118 are input to peak detectors 120 and 121, respectively, and output voltages from the peak detectors 120 and 121 are input to a differential amplifier 124. Likewise, output voltages of the clamp circuits 117 and 119 are input to peak detectors 122 and 123, respectively, and output voltages from the peak detectors 122 and 123 are input to a differential amplifier 125. The differential amplifiers 124 and 125 are also supplied with offset voltages output from offset voltage generators ill and 112, respectively. The differential amplifier 124 outputs a torque signal S.sub.T1, an output voltage representing the detected torque; the differential amplifier 125 outputs a torque signal S.sub.T2, an output voltage representing the detected torque.
In the torque sensor of the above configuration, when the oscillator 110 is energized, the oscillating voltage produced is applied through the buffer circuit 113 to the torque detecting coil 114 and the temperature compensating coil 115. As a result, the torque detecting coil 114 generates an AC voltage which varies with the ambient temperature as well as with the torque exerted from the steering wheel; the AC voltage thus generated is supplied to the clamp circuits 118 and 119. On the other hand, the AC voltage generated in the temperature compensating coil 115 varies with the ambient temperature and is supplied to the clamp circuits 116 and 117.
The clamp circuits 116, 117, 118, and 119 each clamp the voltage by applying such a bias that the voltage waveform is shifted to the positive voltage side until the maximum negative value of the input AC voltage becomes equal to 0 V. The output voltages of the clamp circuits 116 and 118 are fed to the peak detectors 120 and 121, respectively, while the output voltages of the clamp circuits 117 and 119 are fed to the peak detectors 122 and 123, respectively. Each of the peak detectors 120, 121, 122, and 123 detects the peak value of the input AC voltage.
The output voltages of the peak detectors 120 and 121 are input to the differential amplifier 124, and the output voltages of the peak detectors 122 and 123 are input to the differential amplifier 125. The differential amplifier 124 amplifies the difference between the voltages applied from the peak detectors 120 and 121, and adds the offset voltage to the result. Likewise, the differential amplifier 125 amplifies the difference between the voltages applied from the peak detectors 122 and 123, and adds the offset voltage to the result. Accordingly, the differential amplifiers 124 and 125 produce output voltages respectively offset against the voltage associated with the ambient temperatures of the torque detecting coil 114 and the temperature compensating coil 115, thus obtaining two output voltages, i.e. the torque signals S.sub.T1 and S.sub.T2, proportional to the torque applied. The offset voltages are applied to set the torque signals, S.sub.T1 and S.sub.T2, to the midpoint of the torque signal when no torque is applied.
Therefore, when no torque is applied, the output voltages of the differential amplifiers 124 and 125 correspond to the respective offset voltages; on the other hand, when a torque is applied, the output voltage changes in increasing or decreasing direction with respect to the offset voltage according to the direction of the applied torque, based on which the steering direction and the applied torque are detected. The torque signals S.sub.T1 and S.sub.T2, proportional to the detected torque, are then applied to a power steering control circuit (not shown). Based on one or other of the applied torque signals, S.sub.T1 or S.sub.T2, the power steering control circuit controls the rotation of the motor to assist the steering force. If the voltage difference between the torque signals S.sub.T1 and S.sub.T2 becomes greater than a prescribed value, then the situation is determined as an erroneous detection of the torque due to a failure in the torque detected, detector, and control of the motor to assist the steering force is stopped.
When a torque is being exerted by steering action at the steering wheels, if the oscillating voltage disappears due to a failure of the oscillator 110, the AC voltages both in the torque detecting coil 114 and the temperature compensating coil 115 will also disappear if this happens, the torque signals being output from the differential amplifiers 124 and 125 both return to the offset voltage, i.e. the midpoint voltage of the torque signal which is normally obtained when no torque is applied, and as a result, control of the motor rotation to assist the steering force is turned off. Even when [he oscillator 110 fails, the power steering control circuit continues to operate with the output voltages of the differential amplifiers 124, 125 both set at the midpoint voltage of the torque signal. If the oscillator 110 resumes normal operation due to some cause, the differential amplifiers 124 and 125 again output torque signals proportional to the torque being exerted, based on which signals the rotation of the steering assisting motor is controlled. Thus, in cases in which the oscillator 110 intermittently fails, control of the motor rotation to assist the steering force becomes intermittent, which hampers stable steering operation.